1. Field of the Invention
This invention relates to the small-sized variable resistor which is used for a hearing aid, a measuring device, a communication apparatus, a sensor, in addition to industrial apparatuses, etc.
2. Description of the Related Art
In recent years, the reduction of size and weight of various apparatuses has been demanded. Among them, the size of hearing aids has become smaller. The progression has been from the pocket type to the ear suspending/hanging type, and further to in an ear canal type (canal type). In the ear canal type, a person can wear the main body of the hearing aid in the ear canal. In the ear canal type hearing aid, smaller components are particularly required. Moreover, in the case of the hearing aid of an ear canal type, the humidity is high when it is worn. Since there is a possibility that sweat may permeate inside the hearing aid and cause a failure, a waterproof and moisture-proof structure is desired for a variable resistor which is used in the apparatus.
The applicant has disclosed a reliable micro variable resistor. See, for example, Japanese Patent Application No. 9-157741, Japanese Patent Application No. 9-365166. FIG. 1-3 of the present application shows an example of it. This variable resistor comprises a case 1, a resistance substrate 2, a rotor 3, a slider 4, and a shaft 5 for external operation. On the upper surface of the case 1, a circular opening hole 11 and a stepped surface 12 on the periphery thereof are formed. The stepped surface 12, on which an O ring 6 is disposed, has an annular ring shape. Further, a recess 13 in the shape of an annular ring, which a part of the shaft 5 fits, is formed on the periphery of the stepped surface 12. An internal space 14 of a cylindrical shape contains the rotor 3 and the slider 4 and is formed on the inside of the case 1. A stopper part 15 (FIG. 3) protrudes into the internal space 14.
As shown in FIG. 1 the resistance substrate 2 defines a rectangular plate which fits to an opening 16 of the case 1 at the lower end. Sealing resin 20 is injected in the gap between the resistance substrate 2 and the lower-end opening 16, and in the hole at the base of the resistance substrate 2 (FIG. 3). The lower-end opening 16 of the case 1 is thus sealed. A first terminal 21 is inserted in the resistance substrate 2 toward the central portion of the resistance substrate 2 from one side. Second and third terminals 22 and 23 are insert-molded to project toward the central portion from the opposing side. Recesses 19a, 19b, 19c in lower edges 18 of the case 1 receive the first through third terminals 21, 22, 23 when the resistor substrate 2 is sealed to the bottom of the case 1.
One end of the first terminal 21 is exposed to the surface of the central portion of the resistance substrate 2 forms a collector electrode 25. One of the end parts 22a and 23a of each of the second and third terminals 22 and 23 are also exposed to the surface of the resistance substrate 2. A circular resistor 24 is formed on the exposed electrode end parts 22a and 23a. The resistor 24 is formed concentrically on the periphery of the collector electrode 25.
A sleeve 31 protrudes from the middle of the rotor 3 at the upper surface. The sleeve 31 is inserted into the opening hole 11 of the case 1 and is rotatable. An insertion hole 32 is formed so as to penetrate the central part of the sleeve 31 along its axis. A notch groove 33 is formed in the diameter direction on the upper end of the sleeve 31. A disc-shaped flange portion 34 is formed on the lower part of the rotor 3. A stopper part 35 protrudes in the radial direction from the flange portion 34. When either side of the stopper part 35 abuts both sides of the stopper part 15 of the case 1 (FIG. 3), the rotation angle of the rotor 3 is limited. A recess for fitting the slider 4 is formed on the bottom surface of the rotor 3, particularly the bottom of the flange portion 34 and the stopper part 35.
The slider 4 is made of the conductive metal plate having springy or elastic property, such as a copper alloy, a stainless steel, and a precious-metal group alloy. A base portion 41 is provided as part of the slider 4 in the central section so as to be attached with the rotor 3 with the aid of tabs 44 on two edges of the slider 4. An approximately U-shaped first arm portion 42 is formed on one end of the base portion 41. A second arm portion 43 is formed on the other end. The first and second arm portions 42 and 43 are respectively bent in the opposing direction to form bent portions 42b and 43b. A first contact-portion 42a (FIG. 3) slidably contacted with the circular resistor 24 is formed on the end of the first arm portion 42. A second contact-portion 43a (FIG. 3) having a hemisphere surface which contacts the collector electrode 25 is integrally formed on the end of the second arm portion 43.
The shaft 5 has a large diameter operating part 51 on one end. A groove 52 for engaging a driver is formed in the diameter direction on the surface of the operating part 51. A small diameter axial part 53 protrudes at the other end of the shaft 5 so as to be inserted into the insertion hole 32 of the rotor 3. Two projection parts 54 are formed on the sides of the axial part 53 symmetrically to be engaged with the notch groove 33 of the rotor 3.
The assembly of shaft 5 to the above-mentioned case 1 is performed as follows. The O ring 6 is arranged on the step surface 12 of the case 1. The axial part 53 of the shaft 5 is inserted into the insertion hole 32 of the rotor 3 contained inside the case 1. The projection part 54 of the axial part 53 fits into the notch groove 33 of the rotor 3. Thus, relative rotation in relation to the shaft 5 and the rotor 3 is prevented. The rotor 3 and the shaft 5 are integrated by crimping and expanding an end portion of the axial part 53 which projects to the lower surface of the rotor 3. A crimped portion 53a is shown in FIG. 3. In this state, the O ring 6 is pressed and maintained between the internal surface of the operating part 51 of the shaft 5 and the bottom of the step surface 12, the gap between the shaft 5 and the case 1 is sealed.
As mentioned above, when the O ring 6 is pressed in the axial direction between the shaft 5 and the case 1, stress to loosen the engagement of the shaft 5 and the rotor 3 effects by the resiliency of O ring 6. In the case of a small-sized variable resistor, reflow soldering is used commonly as for the mounting method to a circuit board. However, under the condition that the resiliency of O ring 6 exists, heat of the reflow soldering tends to generate a heat transformation, because heating is conducted under high stress and the heat-resistant temperature of components becomes lower. This means that the heat of the reflow soldering causes the shaft 5 to float and an unnecessary gap to form between the rotor 3 and the shaft 5. There was a possibility that a reduction of the air-tightness, or hermeticity, might be caused. Moreover, since electric insulation is required, the rotor 3 and the shaft 5 are often formed with a heat resistant resin. Among them, when a thermoplastic resin which has good fabricability and workability is used, there was a problem that it was easy to receive the influence of the above-mentioned heat.